TABLE OF CONTENTS The nickel-iron battery was invented by Thomas Edison in 1901 to provide electric cars with a longer range than was possible with the lead-acid batteries of the time. Interest in electric cars was high, as they had more than 50 per cent of the then expanding car market (see Chapter 2). With the subsequent virtual disappearance of the electric car, the nickel-iron battery came to be used extensively as a reliable, long-lived but expensive commercial secondary battery in stationary applications.
The battery uses nickel as the positive electrode and iron as the negative, usually with a potassium hydroxide electrolyte. The high exchange current of hydrogen on iron results in considerable gassing, particularly during charging, and also causes corrosion of the iron electrode. This shortens the battery life when not in use by self-discharge. Iron oxides and hydroxides are also transferred to the nickel electrode by ion diffusion. This self-discharge can be quite large, with over 5 per cent of the energy stored in a fully charged battery being lost after only four hours, although the loss rate does decrease as the state of charge reduces. Adding sulphur to the electrode, or lithium, sulphide ions, or hydrazine sulphate to the electrolyte inhibits these processes.
In recent developments energy density has been increased from the 25 Wh/kg originally obtained in stationary batteries, to 50 Wh/kg in prototype development batteries, this improvement being achieved by better utilisation of the active materials and by weight reductions of the inactive battery components. Action...
